1. Field of the Invention
The present invention relates to a method of realizing hard handoff between frequency assignments (FA), and more particularly to a method of realizing hard handoff between FA in a code division multiple access (CDMA) mobile communication system.
2. Background of the Related Art
Generally, in a CDMA mobile communication, all base stations share the same frequency and each base stations has sectors utilizing an independent PN offset. Also, the probability of call drops for a mobile station which moves between the base stations having different FA is relatively high. Thus, the CDMA mobile communication allows a number of respective base stations to have the same number of FA within overlapping zones of operation in which the mobility of a mobile station is frequent.
When a mobile station moves from one base station to another, a handoff allows a continual communication by switching radio lines to connect speech paths between the base stations. Thus, a communication network can be constructed using a limited frequency resource thereby increasing the frequency efficiency. Particularly, there are three types of handoff, soft, hard handoff and idle.
A soft handoff occurs when a mobile station moves from one base station to another base station wherein the base stations have a common FA. In the soft handoff, a communication with a new base station begins before communication with a base station currently in service ends. FIG. 1a shows a soft handoff between Base Station A and Base Station B. A mobile station moving from Base Station A to B simultaneously communicates with both Base Station A and B within a boundary area joining the operation zones of the base stations. If the mobile station continues to move from Base Station A to B. the signal from Base Station A to the mobile station becomes too weak for an effective communication. Thus, the mobile station ends communication with Base Station A and begins communication only with Base Station B, which also ends the handoff procedure.
A hard handoff occurs when a mobile station moves from one base station to another wherein the base stations have different FA. In the hard handoff, a communication with a new base station ends before communication with a base station currently in service starts. FIG. 1b shows the hard handoff between Base station A and Base Station B. A mobile station moving from Base Station A to B ends communication with Base Station A at the boundary area joining the operation zones of Base Station A and B. Afterwards, the mobile station begins to communicate with Base Station B which ends the handoff procedure. Because the signals of corresponding base stations are weak at the boundary area between the base stations, the probability of call drop is high during the hard handoff. Accordingly, the mobile station communicates with a new base station after ending communication with the previous base station.
An idle handoff occurs while the mobile station is in a standby mode, whereupon the mobile station is not actively communicating with a base station. In the idle handoff, the mobile station receives from the base station an overhead channel which includes a pilot channel, a synchronizing channel, and a paging channel. The mobile station also receives a system parameter or paging from the overhead channels to realize the idle handoff. If the mobile station moves within the boundary area where a number of different FA are utilized, the mobile station stays synchronized to the current FA until the mobile station receives information for the new list of FA. Unless the mobile loses synchronization to the current FA, the boundary area remains as an uncompleted service area.
Moreover, for the standard IS-95 of a CDMA cellular system or the standard J-STD-008 of CDMA PCS systems, the frequency band of 1.25 MHZ is utilized as one FA. One FA is utilized in one sector of a base station allowing simultaneous communication by 15 to 30 mobile stations. Also, a number of FA are commonly utilized by all base stations within a designated area and additional FA may be utilized in response to a traffic load. When a mobile station moves within base stations having the same FA, the soft handoff is possible. However, the hard handoff is performed rather than a soft handoff when a mobile station moves between base stations having different FA.
Furthermore, subscribers in downtown areas are high causing a high traffic load. To accommodate the high number of subscribers and Lo increase the traffic load capacity, macro-cell base stations or micro-cell base stations are additionally built in a traffic congestion area. It is preferable to build a micro-cell base station because the service coverage of an additional base station should be limited in the traffic congestion area.
If building additional base stations is difficult, the subscriber capacity can be increased by increasing the number of FA for all the stations in the same zone of operation. However, the amount of overloaded base stations having a traffic load greater than the service capacity is relatively low in downtown or university towns. On the contrary, most of the base stations have a traffic load smaller than the service capacity. Therefore, an overall increase of the FA number for all the base stations in the same zone of operation is uneconomical.
FIG. 2 shows the configuration of sectors in base stations in the related art with Pilot Beacon which assists the hard handoff between base stations having different FA. As shown, in a CDMA mobile communication system with base stations consisting of three interconnected sectors, there are traffic congested base station and neighboring base stations. The traffic congested base station utilizes both the common FA shared with the neighboring base stations and additional FA, while the neighboring base stations utilizes only the commaon FA shared with the traffic congested base station.
Moreover, among the sectors of the neighboring base stations, the sectors adjacent to the traffic congested base station are set as sectors which utilize the pilot beacon. For example, if the traffic congested base station utilizes a first and a second FA, and if a neighboring base station utilizes a third FA, a boundary sector among the sectors in the neighboring base station utilizing a portion of FA common to the traffic congested base station is set as the sector utilizing the pilot beacon.
The pilot beacon was suggested by Qualcomm of the United States to manage the pilot beacon by mounting an overhead channel in different FA, i.e. the additional FA, rather than the commonly utilized FA. Also, excluding a traffic channel, the overhead channel includes a pilot channel, a synchronizing channel and a paging channel.
If the mobile station moves from an area with a large number of FA to an area with a small number of FA and if the area with a large number of FA is utilizing an additional FA, the mobile station within the boundary area transmits a pilot strength measurement message (PSMM) requesting a handoff procedure to a base station currently in service. In response, the hard handoff is implemented by the common FA at the boundary area of the base station currently in service. Subsequently, the mobile station implements a soft handoff to a neighboring base station.
Meanwhile, to enable the idle handoff, the mobile station is tuned to the common FA by receiving a global service redirection message (GSRM) in a paging channel of FA corresponding to the pilot beacon. Afterwards, a new FA among a number of common FA is computed through a CDMA channel list message so that the mobile station is retuned to the final FA.
The method hard handoff procedure as describe above has several problems. Since the hard handoff between FA is implemented in the boundary area between the base stations whose signals are weak, the probability of message loss is high causing a high probability of call drops. In addition, since the soft handoff is implemented utilizing the common FA immediately after the hard handoff is implemented to the common FA in the base station, the common FA in a overloaded sector has a more serious overload than the additional FA, thereby causing a traffic inbalance among the common FA and additional FA.